This invention relates to a ground-based antenna for satellite communications systems using geosynchronous earth orbit satellites and, more particularly, relates to a ground-based antenna that reduces interference from selected satellites by aligning the nulls of the antenna radiation pattern with the interfering satellites.
The rapid growth of the Internet and the unavailability of high-speed connections from telephone lines and local cable providers have resulted in an intense search for an alternative high-speed mode of communications. Satellite communications (xe2x80x98SATCOMxe2x80x9d) systems are a natural selection for replacing conventional land-based communications systems as a means of providing high-speed digital communications links. Furthermore, the broadcast feature of SATCOM systems (i.e., transmitting a signal from a single ground terminal to a satellite for retransmission to a large number of ground terminals over a wide area) also has great potential to the Internet for applications such as xe2x80x9cmulti-castingxe2x80x9d and E-Commerce to a large number of users over a wide area.
Typically, several major obstacles must be overcome to implement a SATCOM system on a wide scale. First of all, antennas used for SATCOM systems tend to be expensive and large to insure that adequate signal strength is received. For example, for an antenna to be able to receive a downlink signal from the satellite, the antenna aperture must have sufficient gain to provide the requisite signal-to-noise ratio to support the required data rate and modulation format. Furthermore, the antenna aperture must be sufficiently large in order to provide a link margin to adequately compensate for environmental effects such as rain, pointing errors, and component degradation.
To transmit an uplink signal, similar design problems exist. For example, the ground-based terminal must use sufficient transmitter power to support the data rate and modulation format and meet the required link margin. However, it is desirable to use the lowest possible transmitter power because the power amplifier is the most expensive component in the ground terminal. Therefore, transmitting at a high power output greatly increases the cost of the ground terminal. One way to keep the cost of the ground terminal low and still meet the link margin is to use an antenna with a large area, and therefore, a high gain. However, this solution is unacceptable to individual users, who demand smaller antennas.
For example, the recently launched ASTRA 1H satellite can receive uplink data at rates up to 2 million bits per second (Mbps) from ground-based terminals. However, in order to achieve these high data rates, the ground-based terminals require antennas with a 48-inch diameter operating with a 2-watt transmitter. Typically, these large antennas are marketed to commercial users due to their large size and high cost. These large diameter antennas are unacceptable to individual consumers due to their large size, which can be obtrusive in a residential community (and in some instances,violative of restrictive covenants) and their high costs.
In an attempt to make SATCOM antennas more appealing to the individual user, makers of SATCOM systems have started to exploit the Ka band of the radio frequency spectrum. In comparison to prior SATCOM systems, operation of a ground-based antenna at the Ka frequency band can result in a reduction in antenna size and decreased power requirements. For example, a SATCOM system using a frequency of 28 GHz (Ka band) for the SATCOM uplink, as opposed to the more typical 12 GHz (Ku band), can reduce the size of the antenna by approximately 43% (12/28). Thus, using the Ka frequency band instead of the Ku frequency band for communicating with a GEO satellite brings the size of the antenna required for SATCOM use more in line with the individual user""s expectations.
To ensure that the SATCOM system provides complete coverage of the continental United States (xe2x80x9cCONUSxe2x80x9d), SATCOM systems employ multiple GEO satellites spaced evenly along the equatorial orbit. For example, a system may station a satellite every two degree (2xc2x0) along the equatorial arc over CONUS. The close spacing of the GEO satellites causes the ground terminal to produce undesireduplink interference to adjacent GEO satellites in the equatorial arc. Additionally, the carrier-to-interference (C/I) ratio of the downlink signal is also degraded due to interference from the adjacent satellites stationed in the equatorial arc. The source of the interference is mostly attributable to the radiation pattern of the ground terminal, and more specifically, the strength and direction of the sidelobes of the radiation pattern.
Historically, most manufacturers of antennas have attempted to solve the problem of interference by using a terminal configuration consisting of an offset reflector feed horn in conjunction with a parabolic dish antenna. This configuration produces a xe2x80x9creasonablyxe2x80x9d low sidelobe radiation pattern resulting from the tapered illumination of the parabolic dish. However, the parabolic dish typically causes the radiation pattern to xe2x80x9cbroaden,xe2x80x9d thereby reducing the overall gain of the antenna. In order to reclaim the reduced gain, the size of the parabolic dish must be made larger and therefore, more expensive. The increased size of the antenna increases the strength of the sidelobes, usually beyond the allowable power limits set by regulatory bodies, such as ETSI. To reduce the power transmitted in the sidelobes while still maintaining the required antenna gain, the phase and amplitude errors over the entire parabolic dish must be precisely controlled. This increases the complexity and expense of the ground terminal beyond a cost that most individual consumers are willing to bear.
Thus, there is a general need in the art for small, inexpensive ground-based antennas for SATCOM systems, which operate in the Ka frequency band and meet regulatory body standards, such as ETSI standards. There is a further need in the art for ground-based antennas, which when pointed at the Nth GEO satellite in an equatorial arc, reduce the interference from adjacent satellites within the equatorial arc.
The present invention meets the above-described needs by providing a ground-based antenna having a uniquely shaped aperture for communications with geosynchronous earth orbit satellites. The uniquely shaped aperture allows the ground-based antenna to maximize the received signal from a GEO satellite in which the ground-based antenna is communicating with while minimizing interference from adjacent GEO satellites.
Generally described, the invention provides a ground-based antenna producing a circularly polarized, far-field radiation pattern having a main lobe and several side lobes. The main lobe points along a transmission axis from the antenna to a first GEO satellite within a constellation of GEO satellites. Each of the satellites in the constellation is equally spaced along an equatorial arc around the earth.
The ground-based antenna contains a uniformly illuminated aperture, which is typically defined by a pair of horizontal side members having a first dimension, and a pair of vertical side members having a second dimension. The horizontal and vertical side members can be connected to form a rectangular aperture lying within a plane perpendicular to the transmission axis.
The horizontal dimension of the aperture is selected so that the nulls of the radiation pattern in the horizontal direction are directed at GEO satellites that are adjacent to the GEO satellite that is positioned within the main lobe of the antenna pattern is directed towards. Furthermore, the second dimension of the pair of vertical members corresponds to the gain of the antenna.
More specifically described, the invention provides a ground-based nulling antenna having a frequency selective-surface covering the aperture. The frequency selective surface has a refractive index that only allows the transmission of a signal having a predefined frequency to pass through to the aperture. Typically, the single frequency will correspond to the uplink frequency for transmission to the GEO satellite. The frequency selective surface reflects all other frequencies.
The frequency selective surface can be shaped as a parabola-shaped reflector. The parabola-shaped reflector may be skewed about the first transmission axis so that reflected energy is directed along a second transmission axis into a feed horn receiver.
The invention may also use a number of dual-polarized radiators to provide a uniform illumination of energy upon the aperture. Each dual polarized radiator is excited by a feed network that is capable of producing a signal having a single polarization or producing two signal having different polarization states. For example, the feed network may produce a single signal that is right-handed circularly polarized. Furthermore, the feed network may produce two signals, each signal having different polarization states, such as a right-handed circular polarization and a left-handed circular polarization.
The aperture of the ground-based antenna also can include a top member with a geometrically-tapered contour. The geometrically-tapered top member has a contour that is calculated to allow each of the nulls form the radiation pattern to fall on at least one of the geosynchronous earth orbit satellites adjacent to the first satellite at which the aperture is pointed. In the alternative, the geometrically-tapered dimension can include the contour of a Gaussian function.
The antenna can include an additional member that bisects the aperture into two separate half-apertures of equal dimensions. One of the half apertures is inverted about the horizontal plane relative to the other half aperture.
The invention may also provide a uniformly-illuminated second aperture comprising a pair of horizontal side members having a first dimension, a top members having a second dimension, and a bottom member having the same geometrically-tapered dimension as the first aperture. The second aperture is placed in close proximity to the first aperture. For example, the bottom member of the first aperture may be placed in contact with the top member of the second aperture. In this manner a larger geometrically tapered aperture that is symmetrical about the horizontal plane may be produced.
The invention also provides for a method for directing a ground-based antenna having a radiation pattern comprising multiple of nulls and lobes generated from a uniformly-illuminated aperture, the uniformly-illuminated aperture having a horizontal dimension and a vertical dimension, on a plurality of geosynchronous earth orbit satellites equally spaced by an angle subtended by the center of the earth. The first step of the method is to make an initial calculation of the horizontal dimension of the aperture so that each of the nulls of the radiation pattern fall on at least one of the geosynchronous earth orbit satellites. The initial calculation of the horizontal dimension is based on the theoretical assumption that the antenna is located at the center of the earth. The theoretical assumption is required because the angle subtended by two adjacent geosynchronous earth orbit satellites is referenced to the center of the earth.
Because a ground-based antenna is located on the earth surface, the angle subtended by two adjacent satellites relative to the ground-based terminal is greater than the theoretical angle subtended by the center of the earth. Consequently, the nulls of the radiation pattern will not be aligned with the interfering satellites in the equatorial arc. Therefore, a correction factor based on the location of the ground-based antenna on the earth surface is required to align the nulls with the GEO satellites. The correction factor can be calculated on a determination of the latitude and longitude of the ground-based antenna. The correction factor is found by determining the degrees of rotation of the aperture about a transmission axis extending normally from the aperture to a geosynchronous earth orbit satellite. Finally, the aperture is rotated about the transmission axis by the calculated degrees of rotation.
Finally, the invention provides a satellite communications system comprising geosynchronous earth orbit satellites that are equally spaced along an equatorial arc around the earth and a ground-based antenna for communicating with at least one of the geosynchronous earth orbit satellites. The ground-based antenna produces a radiation pattern at a first frequency and a first polarization comprising a series of equally spaced lobes separated by a series of nulls. The center lobe is pointed along a transmission axes to the geosynchronous earth orbit satellite, which the ground-based antenna is communicating with. The ground-based antenna has a first dimension that is selected such that nulls of the radiation pattern are aligned with the geosynchronous earth orbit satellites that are adjacent to the first satellite in the equatorial plane.